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You could be having hormonal
disturbance which is causing this. Are you taking contraceptive
pills. Avoid sun exposure. Wear sunblock of SPF25 or above only. Eat plenty of fresh fruits and vegetables which have antioxidants to prevent skin damage. Drink plenty of water. You can consult Dermatologist
who will examine you and probably give skin bleaching creams like hydroquinone
which will lighten the mark.